Increase Cloud Evolution Speed?
Global time is basically just the frame number. As far as I know, Blender can't render in between frames, so there isn't a way to slow down global time.

(I may be wrong about that though. Maybe turbulence is computed as a current_frame / total_frames? In that case, you could try increasing/decreasing the number of frames. I don't have time to do a test case, at the moment.)

Instead, you should use "particle age" for the influence. To modify slow down the turbulence, you'll need to select the particles option when you first generate the cloud. This will let you edit the particle system associated with the point cloud texture. Longer particle lifetime = slower; shorter particle lifetime = faster.

See the Blender Wiki for the Point Density Texture

Edit:
Oh, you could also just render it at slow speed, then speed it up using a "speed control" effect in the video sequencer. 'Tis a hack, but may suit your situation.